Method of heat treating cast iron



1959 I L. SAIYES 2,901,386

METHOD OF HEAT TREATING CAST IRON Filed Feb. 6, 1953 IM W m E I -i- 500 I NUMBER OF GRAINS P67? 50. MM.

C 100 20 300 400 '00 600 700 NUCLEA T/QN TEMPERATURE /N "C ms PER. 50 MM. 5 e.

C 100 200 100 400 500 600 700 m! NUCLEA T/OIV TEMPERATURE IN C NUMBER OF GRAINS PER 30. MM.

I I TREATMENT DURATION IN HOURS 2,901,386 Patented Aug. 25, 1959 United States Patent ,Ofitice METHOD OF HEAT TREATING CAST IRON Leon Saives, Billancourt, France, assignor to Regie Nationale des Usines Renault, Billancourt, France This inventionrelates to a process for the controlled graphitisation treatment of white cast iron which can be rendered malleable and pearlitic, applicable to the usual compositions of cast iron which can be rendered malleable and in particular to cast irons having an ,ratio suitable for obtaining white castings when cast in sand and in metal chill-moulds despite the thickness of the pieces, applicable to the compositions rich in sulphur and phosphorus which result from the use of current raw materials and cupola-smelting, wherein after being cooled the castings are subjected to a heat treatment in three successive phases under precise conditions. A

Technical literature indicates that the malleable cast irons, that is to say the white cast irons (cementite structure), annealed in the solid state to obtain the graphitisation of the carbon, may show a finer distribution of the graphite, that is to say have smaller grains in greater numbers, ifthe heat treatment applied to the white, casting comprises either a martensitic hardening prior to the annealing, or a more progressive reheating for the annealing, or, a preliminary hardening followed by progressive reheating. Nevertheless, up to date, it has scarcely been possible to utilize these results outside the laboratory, because there wasno known method of obtaining this fine structure regularly, or of applying it to the impure castings produced under industrial conditions, or of applying it to industrial. castings of variable, and sometimes substantial, thickness,

In fact, all the experiments reported relate to the use of cast iron having very low manganese content and high carbon or silicon contents; the proportionsby weight are:

0 less than 0.14

and

less than 0.27

with the result that these cast irons have a strong tendency towards graphitisation from the moment they are cast and if the speed of solidification and cooling is sufiicient to obtain a white structure, they are easy to graphi tise by annealing. In order to obtain this white structure, the test-pieces cast have very narrow diameters, less than 7 mm., which, as a result of the rapidity of solidification, causes a central shrinkage hole, preventing high mechanical properties from being obtained. Moreover, using these compositions, if normal parts such as are found in mechanical construction were cast in sand, that is to say parts one or more centimeters thick, the casting would be irrevocably mottled, or even completely grey and unsuitable for obtaining parts with great toughness or malleability.

Moreover, it has only been possible to obtain these results with cast irons prepared in an electric arc furnace having the composition:

a 2 or high frequency furnace, starting with very pure raw materials, in order to have very low sulphur and phosphorus contents of less than 0.05% each, which is incompatible with the use of the normal foundry raw materials and the normal procedure of re-smelting in a cupola furnace, because of the sulphur absorbed from the coke,

It has only been possible to perform the preliminary martensitic hardening satisfactorily, in the experiments carried out to date, because of the small dimensions of the test pieces and their regular shape, and moreover micro-cracks frequently occur, to such an extent that some investigators have claimed that these cracks were the cause of the multiplication of the grains of graphite observed in the course, of their experiments. It is manifestly impossible to heat parts made of white cast iron to the temperatures of 870 C. to 950 C. indicated by these authors and to harden them in oil without a serious risk ofshrinkage cracks, which renders the process unusable onanindustrial scale. a

According to the invention, a very fine distribution of the nodular graphite produced by annealing can be obtained with very great regularity in industrial sand-castings containing sulphur and phosphorus contents of between 0.05% and 0.15% each, so that the cast irons of current quality can be re-smelted in a cupola furnace and the finishing touches given to the composition in a reverberatory furnace, which constitutes the most highly recommended industrial process for the preparation of castirons which maybe rendered malleable. These cast irons have ratios:

greater than 0.17

and

greater than 0.36

so that the sand-casting of mechanical parts having a thickness of the order of 1.5 to 3 cm. leads to a perfectly white structure; this is the case with the cast iron C=2.35%, Si=l.l5%, Mn=0.40%, S=0.12%, P=0.10%

of the type currently used in malleable iron founding. The process may equally well be applied to ordinary malleable cast irons and to special cast irons.

The applicant has established the fact that in order to obtain, by heat treatment, a fine distribution of the graphite in White iron castings of the usual thickness of between 0.5 and 3 cm., and thus to reduce the duration and the temperature of the graphitisation of the primary cementite,,precise new conditions of treatment, constituting the main object of the present invention, must be observed. Essentially this treatment comprises three successive phases:

(1) Preliminary martensitic hardening,

( 2) Tempering to induce the formation of graphite nuclei,

(3) Annealing to produce graphitisation of the primary cementite;

of shrinkage cracks or of deformation, and at a modcrate price, if the pre-hardening austenisation is carried out just above the point where the pearlitic translie between 24 hours and 48 hours. I white cast iron of the ordinary composition, this optimum a precision of i.

formation ends during heating; in practice, a margin of 20 C. to 50 C. may be allowed; the temperature of 810 C. is suitable in the example quoted. The period of maintenance at this temperature is that normally observed for hardening, that is to say, the total heating period may be of the order ofhalf an hour. Instead of quenching in water or oil, the hardening is carried out in stages in a suitable liquid such as a salt-bath and without risk of shrinkage cracks. The temperature and the duration of the immersion are such that no transformation takes place during the immersion; this temperature is therefore only very slightly above Ms, the temperature at which the martensitic, transformation begins; it is generally between 150 and 250 C. The period of maintenance is such that no initial transformation takes place in the bath; according to circumstances, the time may be between seconds and 3 minutes at 225 C., or between 1 minute and 6 minutes at 175 C.

In the example chosen, satisfactory results are obtained by immersion for 1 minute in a salt-bath at 180 C. This hardening is followed by cooling at room temperature, preferably in still air.

The tempering to decompose the martensite and initiate out. It has been shown that for ordinary malleable cast irons cast in sand, very precise conditions must be observed, which are not necessary in laboratory experiments carried out on small test-pieces which are solidified the nuclear formation of graphite is then carried a very rapidly, and the composition of which "permits o'f simple graphitisation by annealing. On the other hand with the ordinary white cast irons, despite the thickness of the pieces, it is important to subject them to a tempering process which is also a treatment for the nuclear formation of the graphite because it is essentially on this that the number of nodules of graphite formed later depends; thus from the number of nodules formed in the subsequent graphitis'ation it is possible to deduce the number of nuclei created in the course of this tempering.

In the accompanying drawings, Figures 1 and 2 give 8 this number of nodules per mm. section, as a function of the temperature of the nuclear formation treatment with a constanttemperature duration of 48 hours or 96 hours. It will be seen that a'pr'ecise temperature should be observed, which depends on the'cornposition: 510

C. for a cast iron without any added constituents. Figure 3 shows the effect of the duration of this nuclear formation treatment. It will also be seen that this duration depends on the temperature. If the temperature is suitable, the number of nuclei increases asa function of the time in a asymptotic manner towards one limit. As

the cost of treatment increases proportionally with the time there is. a compromise and the optimum time should Consequently for to date. For "other cast irons, differing in their composition or preparatiomthis precise temperature may be reduced to 450 C.

It is also possible, "according to the invention, to carry out the nuclear formation in two stages, the first at a lowertemperature of the order of 280 to 350 C., the duration of which may vary from 10 to 100 hours, and which is followed by a second phase similar to the treatment above. By way of example, white cast iron which can be rendered malleable, having. been prehardened then treated for 100 hours. at 330 C. then for 48 hours at 510 C., produces 2,200inuclei of graphite per mm. section (the nodules of graphite having transverse ,dimensions of the order of 6, microns), whereas in the nuclear form ation in a singleiphase for 48 hours at 510 .C., the number of nuclei is 1,000 per mmF. From this can be obtained to advantage by progressive reheating and stages.

By way of comparison, the same cast iron, not having been subjected to preliminary hardening and subjected to nuclear formation for 48 hours at 500 C. would produce nuclei per mm. and the same cast iron, subjected to the same graphitisation treatment but not having been subjected to preliminary hardening or nuclear formation treatment, wouldundergo practically no graphitisation; the number of nuclei would be nil; treated under the usual annealing conditions for malleable iron the nodules would have an average diameter of 100 microns and would be very limited in number, 30 to 60 mm.

The complete treatment then comprises an annealing cycle at a high temperature, above the transformation zone for carrying out the graphitisation of the primary cementite. This treatment can be carried out at a constant temperature and requires a shorter time the higher the temperature the greater the number of nuclei. The "speed of graphiti's'ation can be expressed-in the form N being the number of graphite nuclei previously formed per unit of volume and T the absolute temperature, A "and "Q constants varying slightly with the composition. Theq'uality is better when the treatment is limited to the time just necessary, because later 'a coalesiee'nee of the nodules of graphite formed is'noticed, which hasa detrimental effect on the quality; it is therefore adusable to limit the treatment in time and in temperature. Moreover, the cost of the treatment, which, other things being equal, increases with the time and temperature, would scarcely depend on the compromise chosen it werenot necessary to take into account the depreciation the material and tools used for the treatment. Since the raising foffthe "temperature is particularly harmful to the latter, it"should be limited. The "process for co trolled nuclear formation which 'has just been described rnakes it possible,in particular, lto carryout this annealing under "conditions which would not produce any graphit'isation, had there not been the preliminary "double treatmentof hardening and nuclear formation. This i'sfshown by thefollowing tests relating to a cast iron the percentage composition C=2.36, Si=1.15, 'Mn=0;36, S=0.l2, P='0.'10'%; the preliminary harden'ing comprises austenisation for 30 minutes at 810 .After preliminary hardening in salt for 1 minute at 180 C. and nuclear formation for 48 hours at 500 C.,

it is possible to obtain the complete graphitisation of the primary cementite in 12 hours at 895 C. or 3 hours at If it is not desired to obtain a ferritic structureas in ordinary malleable cast iron, but-a structure of lamelar pearlite with a view to obtaining 'ahigh resistance cast iron, this treatment may be terminated by cooling in still air. L

J In these circumstances, it is easy to obtain the following tensile characteristics in machines test-samples:

Elastic limit 45 kg./mm.2 Breaking load 65 kg./mm. I Elongation 3%' The castings thus graphitised maybe hardened and tempered.

The cast iron may contain such constituents as aluminium, titanium and zirconium which facilitate the nuclear formation under the conditions of treatment mentioned above or which, like nickel and molybdenum, facilitate, after the above graphitisation, the effect of the hardening and tempering treatments with a view to modifying the utilisation properties and more particularly the mechanical properties. The effect of the constituents aluminium, titanium and zirconium, used alone or mixed, is particularly effective in the treatments for controlled nuclear formation forming the subject of the invention; the example relates to cast irons cast in sand and having a white structure, the castings having been reheated at 810 C., hardened in salt at 180 C. for 1 minute, cooled in still air, reheated at 450 C., for 48 hours, cooled, then reheated to 895 C. for 14 hours; (cooling being in still air), the following table gives the number N of fine nodules per mm. section:

The principles of the heat treatment may be extended to the castings of white structure obtained in metal chillmoulds, the process then being easier to apply industrially than with sand-castings and making it possible to obtain high mechanical properties.

If, with a given cast iron composition, pieces which are white are obtained by sand casting, the same pieces, or pieces of the same thickness, would be even more sure to be white if chill-cast. Using this latter method of casting, it would be possible to adjust the ratio of the manganese and silicon contents, and possibly the ratio of the manganese and carbon contents, at a lower value than that which is necessary to obtain a white cast iron by sand-casting, other things being equal. The white structure produced during casting can be preserved, without modifying the composition, in more massive chill-castings than sand-castings.

It has been discovered that the structure of chill-castings is generally finer than that of sand-castings and that after the nuclear formation treatment provided by the invention, the nodules of graphite are much finer and much more numerous, although the nuclear formation and graphitisation operations have a much shorter duration.

The value of the new progress achieved will be better understood by the comparison, given as an example, of treatment carried out on the one hand on sand'castings and on the other hand on chill-castings.

In a cast iron having the composition:

0, Si, Mn, S, P, percent percent percent percent percent hours but at 500 (3., the number of nodules observed becomes 3,000 per mmfi. By the'treatment at 500 C. applied for only 12 hours to a cast iron of the same composition and cast in a metal chill-mould and hardened, 15,000 nuclei per mm. are obtained. In certain cases, it is possible to obtain a result of the same order in much shorter times, for example 3 or 6 hours.

The achievement of the graphitisation in the third phase of the treatment is likewise much more rapid, 2 to 3 hours at 875 C. being sufilcient, while 10' hours at the same temperature are necessary for sand-castings.

The advantages derived from metal mould casting from the point of view of mechanical properties, after treatment according to the invention, are shown by the results The widest range of application is another advantage resulting from the fact that homogeneous white structures can be obtained by chill-casting in massive pieces which would remain partly grey in sand moulds.

These advantages, which are associated with the application of the treatment process, are, in addition to the benefits derived, regardless of any treatment, from the use of permanent moulds, simplification of foundry Work, reduction in machining costs made possible by the greater precision in the moulds, etc.

The castings due to the method of treatment disclosed can be obtained in chill-moulds made of ordinary steel or grey iron or special refractory steel. According to the particular circumstances of manufacture, the chillmoulds may be left bare or coated before casting, by means of a brush or spray-gun, with a thin refractory layer, having a silica-flour base for example. The casting may be made in a cold chill-mould or in heated moulds to avoid the cracks and excessive shrinkage which would result from too abrupt cooling. Cores of burned sand, risers, multiple shrinkage heads of core-sand can be connected to the metal chill-moulds.

White castings, due to the process, can also be produced in moulds, the impression of which is constituted by relatively thin parts obtained by the agglomeration of sand by means of an organic plastic material, for example thermosetting.

I claim:

1. A process for the treatment of white cast iron suitable for formation of pearlitic malleable iron which comprises subjecting a casting of said white cast iron after cooling to a sequence of steps which includes effecting martensitio hardening of the casting by austenization with heat at a temperature slightly above that at the end of the eutectoid transformation followed by progressive cooling including quenching in a salt bath maintained between- 250 C., maintaining the casting in the salt bath for a period of time between 30 seconds and 6 minutes, and then cooling to room temperature to produce the martensitic transformation, tempering the thus-produced martensite by heating at a temperature of 450 to 500 C. for a prolonged period of time of 3 to 48 hours, further heating the thus treated iron at a temperature of about 900 C. to effect graphitization of the iron, and cooling to room temperature.

2. A process as defined in claim 1, wherein the final cooling step is carried out in still air.

3. A process as defined in claim 1, wherein tempering is carried out for a period of from 24 to 48 hours.

4. A process as defined in claim 1, wherein the 'graphitization treatment is carried out at :a temperature of "aLbGVe OB 1.

References Cited in the file of this patent "UNITED STATES PATENTS OTHER REFERENCES UNITED STATES PATENT OFFICE Certificate of Correction Patent No. 2,901,386 August 25, 1959 Leon Saives It is hereby certified that error appears in the printed specification of the above numbered patent requlring correction and that the said Letters Patent should read as corrected below.

Column 4-, lines 22 and 23, for the expression Q Mmeread N Aefi' Signed and sealed this 17th day of May 1960.

Attest: KARL H. AXLINE, ROBERT C. WATSON, Attesting Oficer. Oomnissz'oner of Patents. 

1. A PROCESS FOR THE TREATMENT OF WHITE CAST IRON SUITABLE FOR FORMATION OF PEARLITIC MALLEABLE IRON WHICH COMPRISES SUBJECTING A CASTING OF SAID WHITE CAST IRON AFTER COOLING TO A SEQUENCE OF STEPS WHICH INCLUDES EFFECTING MATERNISTIC HARDENING OF THE CASTING BY AUSTENZATION WITH HEAT AT A TEMPERATURE SLIGHTLY ABOVE THAT AT THE END OF THE ENTACTED TRANSFORMATION FOLLOWED BY PROGRESSIVE COOLING INCLUDING QUENCHING IN A SALT BATH MAINTAINED BETWEEN 150-250*C. MAINTAINING THE CASTING IN THE SALT BATH FOR A PERIOD OF TIME BETWEEN 30 SECONDS AND 6 MINUTES, AND THEN COOLING TO ROOM TEMPERATURE TO PRODUCE THE MARTENSITIC TRANSFORMATION, TEMPERING THE THUS PRODUCED MARTENSITIC BY HEATING AT A TEMPERATURE OF 450 TO 500*C. FOR A PROLONGED PERIOD OF TIME OF 3 TO 48 HOURS FURTHER HEATING THE THUS TREATED IRON AT A TEMPERATURE OF ABOUT 900*C. TO EFECT GRAPHITIZATION OF THE IRON, AND COOLING TO ROOM TEMPERATURE. 